The invention relates to a process and apparatus for extruding electrical insulation around the core of an electrical cable and particularly, for electrical cables to be used at relatively high voltages.
It is conventional in the art to extrude electrical insulation material, such as polyethylene and other plastics, over a core which includes at least one conductive wire. For high voltage power cables, the core usually includes a plurality of stranded wires encircled by a layer of semi-conductive material.
To obtain the highest dielectric strength, the insulation material should be free of contaminants, but in the preparation and handling of the material, contaminants inevitably become included with or within the material.
Manufacturers of electrical grade insulation material go to substantial lengths, such as filtering, washing, etc. of the material which is supplied to the cable manufacturer in the form of pellets, to reduce the contaminants to a low level. Such measures increase the cost of electrical grade insulation material as compared to the same material used for other purposes. However, in the handling and shipping of the pellets, from the material manufacturer to the cable manufacturer and in the handling and processing of the pellets at the cable manufacturer's plant, contaminants, such as dust, rust, etc. become mixed with the pellets.
In addition to the quantity of contaminants, which should be kept low, an important factor is the size of the contaminants. Thus, for high voltage, e.g. 150 kilovolts, the size of the contaminants should be kept below 50 microns. At lower voltages the size of the contaminants can be greater, but, preferably, they should not exceed about 200 microns.
It has heretofore been proposed that the insulation material be supplied to the cross-head which applies the insulation to the cable core by three extruders with a filter between the second extruder and the third final extruder to remove contaminants having a size greater than 50 microns. See the paper by J. Midoz presented at the International Conference on Polymer Insulated Power Cables, Mar. 5-10, 1984 and published in "Jicable" 1984. In the first extruder, a twin screw extruder, the insulation material, such as high density thermoplastic polyethylene or cross-linkable polyethylene, is plasticized with introduction of liquid additives. From the first extruder, the plasticized mixture is delivered directly to a pressurizing second screw extruder at the output of which there is a filter for removing contaminants larger than a predetermined size, e.g. 50 microns for cables to be used at 220 Kv. The filtered insulation material is supplied directly to a third screw extruder of long length which supplies the filtered insulation material directly to the cross-head for applying the insulation material to the cable core. If the insulation material is to be cross-linked by the addition of a cross-linking agent, such as peroxide, it is injected into the third extruder.
It will be noted that in such prior art system that three screw extruders were required, a plasticizing first extruder, a second extruder which will provide sufficient pressure to cause the insulation material to pass through the filter and a third extruder which provides sufficient pressure to supply the filtered insulation material to the cross-head and, if the cross-linking agent is injected at the third extruder, to mix the cross-linking agent with the filtered insulation material. Such extruders are relatively expensive, and the third extruder is not a good mixer and has output pressure fluctuations which cause undesirable thickness variations of the insulation on the cable core.
When particles larger than 50 microns are to be removed by the filter, relatively high pressures are required at the filter input, and if a cross-linking agent is used, the cross-linking agent should not be added to the insulation material upstream of the filter to avoid cross-linking during the filtering. Also, there is a large pressure drop across the filter.